This invention relates to rubber and foamed rubber compositions which retain low resilience over a wide temperature range of from -10.degree. C. to 50.degree. C. and are soft at room temperature. More particularly, this invention relates to rubber and foamed rubber compositions which comprise a styrene-butadiene copolymer or a mixture of styrene-butadiene copolymers having a high styrene content, and relatively large amounts of a filler and a plasticizer, and which have improved vibration-attenuating and shock-absorbing properties at frequencies of 0.01-1000 Hz due to their remarkably low hardness, improved energy- and shock-absorbing ability due to their low resilience over a wide temperature range from -10.degree. C. to 50.degree. C., and an increased tensile strength.
A number of rubber compositions which are soft at room temperature are known in the art, including compositions based on polynorbornene as disclosed in Japanese Patent Application Kokai No. 56-74801, compositions based on ethylene-vinyl acetate copolymers as disclosed in Japanese Patent Publication No. 53-3244, and compositions based on urethane elastomers as disclosed in Japanese Patent Application Kokai No. 52-54000. These compositions, however, were inadequate for practical application because some had a resilience as high as 40 to 90% or a reduced tensile strength of less than 50 kg/cm.sup.2, and none of them satisfied both low resilience and high tensile strength. Low-resilience rubber compositions were also proposed which were based on butyl rubber or modified rubbers. However, they were temperature dependent and had increased hardness.
Most conventional sponge rubbers prepared using organic foaming agents are highly resilient. As far as the inventors know, few investigations have been made on low-resilience sponge rubbers. As to foamed materials, some of foamed polyurethanes have low resilience (see Japanese Patent Application Kokai No. 55-137123, and Japanese Patent Publication Nos. 46-34630, 48-38467, and 50-52175, for example), although they have poor shock-absorbing properties and often become unstable with temperature change. In addition, these foamed polyurethanes are known to be inferior in water resistance and heat deterioration to sponge rubbers, and they are also unsatisfactory in actual application.
The current general trend in the rubber industry is toward light weight products. For low-resilience rubber compositions, it is desired to render them light weight to make them more valuable. However, simply foaming conventional low-resilience rubber compositions with the aid of an organic foaming agent will result in foams having increased resilience because the resilience is largely affected by the introduced closed cells containing gases.